Remotely powered circuits do not have a local power supply: i.e. battery, or power supply unit. The energy they require is conveyed to them over the transmission lines to which they are connected.
For a telephone, the energy is conveyed by the telephone line. Depending on the type of telephone, i.e. whether it uses tone dialing or pulse dialing, energy is not always present on the line. With tone telephones, dialing is done by frequency modulation, i.e. by modulating the current on the telephone line. Energy is always present on the line.
For telephones using pulse dialing, each digit (0 to 9) is represented by a corresponding number of pulses, each having a certain duration or a certain characteristic on the telephone line.
A pulse is obtained by short-circuiting the telephone line, which has the effect of making energy unavailable during each pulse. These power supply interruptions are of fixed duration since they satisfy the characteristics required for dial pulses.
The invention is particularly applicable to pulse dialing. The circuitry of the telephone must be capable of accepting power supply interruptions of fixed duration corresponding to dialing. However it should not tolerate power supply interruptions that do not correspond to dial pulse characteristics: e.g., faulty operation, line breaks, etc. Under such circumstances it must be reinitialized. That is why the term "conditional initialization" is used: initialization is conditional on an interruption of power supply lying outside an acceptable range.
The circuitry of such a digital telephone set essentially comprises two electronic portions: a bipolar logic portion for controlling the interface between the telephone set and the telephone line; and a microprocessor for controlling the telephone keypad, in particular, and for sending instructions to the bipolar logic.
In addition, it is the bipolar logic that performs the voltage regulation function for the microprocessor and which controls conditional initialization of the microprocessor (by controlling a reset pin thereof).
During dialing, the bipolar logic short-circuits the telephone line for each dial pulse and for a predetermined length of time, and it puts the output of its voltage regulator into a high impedance state so as to decouple the voltage regulator from the microprocessor. The microprocessor is then powered by discharging a back-up capacitor.
In contrast, the reset pin will be subjected to fluctuations relating to the state of the telephone line. Since it is necessary to prevent the microprocessor (and the telephone circuitry in general) from being reset during dialing, and since it is also necessary to reset the microprocessor during power supply interruptions that do not correspond to dialing, it is the practice to use a set of discrete components, capacitors, diodes, resistance networks, and transistors, for the purpose of stabilizing the state of the reset pin during power supply interruptions of acceptable characteristics (dialing).
This stabilization circuit is bulky and is installed between the bipolar logic and the microprocessor. An object of the invention is to omit this additional stabilization circuit, thereby eliminating problems of bulk and of calibrating the values of its components. Another object of the invention is to make use of items that are programmable so as to provide a conditional initialization circuit that is flexible and adaptable.